The broad objective of this proposal is to use a mouse melanoma model system to define underlying genetic and epigenetic mechanisms regulating differentiation and the changes that lead to malignant transformation. Two malignant clones, one lelanotic and one amelanotic, and one nonmalignant clone, which has become so after continuous growth in the presence of 5-bromodeoxyuridine (1 grams/m1) make up the melanoma system. The non-malignant clone is capable of immunizing mice against challenge with the parent malignant clones. When either malignant clone is mixed with the nonmalignant clone the mixture is nontumorigenic in immunocompetent mice but forms tumors in immunosuppressed mice. At the same time, plasminogen activator activity is suppressed in the mixtures. The mixtures and/or the nontumorigenic clone cause immunocompetent mice to mount a rapid inflammatory response and one aim is to determine the reason for this and the resulting death of the cells. Conversely, we want to know why the tumorigenic cells do not cause this inflammatory response and are able to grow within the host. We plan to use somatic cell hybridization, cybridization and nuclear-cytoplasmic reconstitution as tools to understand the respective roles of nucleus, cytoplasm and interactions between them in these processes. Study of the effects of melanocyte stimulating hormone should give greater insight into the regulation of melanogenesis and growth in these cells. We also plan to continue to study the biochemistry of the cell surface of these clones in order to dissect molecular mechanisms underlying the malignant potential of most melanoma cells and the mechanisms that make the nontumorigenic cells lose this potential in normal mice and become capable of immunizing them against the malignant parent line.